There is a tree so poisonous that rainwater running off it's leaves will burn your skin.
It's called Manchineel and it's a fruit tree native to Florida. However, you probably don't want to eat the fruit! The fruit looks like an apple and it is believed to cause death when eaten. In fact the spanish name for the fruit is "manazanilla de la muerte" or "little apple of death."
The tree is so poisonous that rainfall that touches its leaves will cause somone's skin to blister. Despite this, they are good natural wind breakers and they're used to prevent beaches from eroding. To prevent people from getting burnt, most trees are marked either with signs or red Xs to indicate that they're a dangerous tree.